Molecular hybridization evidence indicated that transcripts of repeated DNA are transported to the cytoplasm of tumor cells and embryonal cells but are nuclear-restricted in normal, adult cells (Shearer and Muckler, 1971). This and other evidence led to the proposal that malignancy and development share a form of RNA processing not used by adult cells (Gillespie and Gallo, 1975). We intend to establish the validity or invalidity of this proposal by using labeled, purified repeated DNA probes in in situ hybridization. We can identify and purify four forms of repeated DNA. These are simple and tandem, complex and tandem, short and interspersed and long and interspersed. We intend to cleave each form from human and mouse genomes with restriction endonucleases and purify the repeated DNA fragments by electrophoresis through gels of agarose. The purified DNA fragments will be labeled by nick-translation or iodination and used for in situ hybridization, scoring cytoplasmic hybrdization as positive. We will examine blood and marrow cells from normal and leukemic humans and mice as well as sectioned tissues from human malignancies and normal mouse embryos.